The following meanings for the abbreviations used in this specification apply:                3GPP The 3rd Generation Partnership Project        BS Base Station        CB Coordinated Beamforming        CoMP Coordinated Multipoint Transmission/Reception        CQI Channel Quality Indicator        CS Coordinated Scheduling        CSI Channel State Information        JP Joint Processing        LTE Long Term Evolution        LTE-A Long Term Evolution Advanced        MIMO Multiple-Input Multiple-Output        MU Multi-User        PMI Precoding Matrix Indicator        SU Single-User        UE User Equipment        
In Long Term Evolution (LTE) and LTE-Advanced (LTE-A), as proposed by the 3rd Generation Partnership. Project (3GPP), single cell single-user (SU-) and multi-user (MU-) multiple-input multiple-output (MIMO) network performance is interference-limited, especially at the cell edge.
Therefore, introduction of the technology of coordinated multipoint (CoMP) transmission/reception has been considered, where in downlink, multiple base stations (BSs) co-operate in scheduling and transmission in order to strengthen a desired signal and mitigate inter-cell interference. So far, two schemes of CoMP, named Joint Processing (JP) CoMP and Coordinated Beamforming/Coordinated Scheduling (CB/CS) CoMP, are mainly investigated in 3GPP. Both schemes aim at enhancing the system performance by coordination among the cells.
In particular, in JP CoMP, two or more BSs transmit simultaneously to a CoMP user, and in CB/CS CoMP the scheduling decisions of neighbor cells are coordinated in order to reduce interference. The CoMP feature is likely to be included in 3GPP LTE specifications from Release 11 onwards.
The initial problem that CoMP is intended to improve is the performance of cell edge users, because especially at cell edges, the performance is interference limited. In ideal CoMP, all cells in the network would cooperate or one control unit would make all centralized scheduling decisions. In practice, cooperation at this level is impossible due to increase in feedback and scheduling/cooperation complexity.
The cooperation has been limited to N cells, defining the cooperation set. Within the cooperation set, JP or CB/CS CoMP is performed. So far, JP CoMP and CB/CS CoMP have been considered to be separate schemes in 3GPP with different feedback, backhaul and coordination demands and also with different benefits.
In this regard, CB/CS CoMP has been considered to be a lighter way of CoMP as only PMI feedback is needed and scheduler only avoids transmitting to directions that potentially cause interference.
On the other hand, JP CoMP does cooperative transmission and has been considered to be the CoMP scheme that gives better gains but requires more feedback and backhaul.
Further information on JP CoMP can be found, for example, in document [1] and further information on CB/CS CoMP can be found, for example, in document [2].
In order to maximize the benefits of CoMP downlink transmission, the cooperating BS set should include those BSs having the strongest channel strength from one User Equipment (UE). However, in practice, the cooperation set of each UE could be very different and overlapping to each other. Those ‘arbitrarily’ overlapping cooperation sets make the scheduling at network side extremely complicated and even inaccessible.
Therefore, some compromise has to be made, such as employing static/semi-static cooperation set layout for all UEs. Apparently, these uniform cooperation sets could not satisfy all UEs' optimal CoMP requirements, and some set-edge UEs cannot be served by his/her own optimal cooperation set. The same “cell edge problem” remains at the set borders independent of whether JP or CB/CS CoMP is performed within the sets.
In the context of homogeneous networks, 3GPP assumes a network deployment where N (e.g., 9) cells are thought of belonging to a cell cluster that is controlled by one centralized control unit. These N cells are divided into several cell cooperation sets with smaller size, e.g. 1-3 cells each. The different ways of dividing the N cells into smaller coordination sets is named CoMP layouts in the following.
The CoMP layout selection has been considered to be either network centric or UE centric. In network centric layout selection, the control unit determines one fixed layout or adapts among different options. In either way, there will be set-edge users that are not served by CoMP.
As mentioned above and also discussed in document [3], the ideal UE centric approach is impractical and signaling overhead and scheduling complexity limit the gains.
A further proposed scheme is described in document [4]. According to this document, the UE could select from two predefined cluster patterns the more suitable one.
In this scheme, two fixed patterns are available and either CB/CS CoMP or JP CoMP is performed